So high-profile is the case that US President Donald Trump said he could intervene if it helped to avoid a further decline in relations between the US and China, which are locked in a trade war.

However, Mr Trump's own officials frowned on the idea, with US Assistant Attorney General John Demers remarking: "What we do at the justice department is law enforcement. We don't do trade."

Foreign ministry spokesman Lu Kang addressed the matter on Thursday, saying the pair were "suspected of engaging in activities that threatened China's national security" and had been put under "compulsory measures", a usual reference to custody.

Mr Lu said the "legal rights and interests of these two Canadians have been safeguarded" and that the two cases were being investigated separately.

Asked if the detentions were in response to Ms Meng's arrest, he said they were part of an "operation taken by China's relevant national security authorities in accordance with the laws".

Mr Lu said Ms Meng's arrest was "wrong practice", adding: "I can point out that, since the Canadian government took the wrong action at the request of the US and took Meng Wanzhou into custody, many Chinese are wondering if their trips to Canada are safe."

Some China analysts were in little doubt about the reason for the detentions.

Guy Saint-Jacques, Canada's former ambassador to China, told Canadian broadcaster CBC: "In China there are no coincidences... If they want to send you a message, they will send you a message."

One Western diplomat in China told Reuters news agency: "This is a political kidnapping."

Michael Spavor, arrested on 12 December, is a businessman based in Dandong, near the Chinese border with North Korea. He has ties to the North Korean government and has met its leader Kim Jong-un many times.

Canadian foreign ministry spokesman Guillaume Bérubé confirmed that Mr Spavor had contacted it earlier in the week because "he was being asked questions by Chinese authorities".

One of his most recent Facebook posts, on Sunday, said he was about to travel to Seoul in South Korea but he did not arrive on Monday as planned.

Mr Spavor runs an organisation called the Paektu Cultural Exchange, which organises business, cultural and tourism trips to North Korea.

He is a regular visitor to North Korea and regularly comments in the media on Korean issues.

He is particularly well known for helping to arrange the visit by former NBA star Dennis Rodman to North Korea in 2013. Rodman is a personal friend of North Korean leader Kim Jong-un.

Ex-diplomat Michael Kovrig currently works for a think-tank, the International Crisis Group (ICG), which has said it is concerned for his health and safety.

He was arrested on 10 December.

Mr Lu said the ICG had not been registered as a non-governmental organisation (NGO) in China and therefore it was unlawful for its staff to work there.

Checks by Reuters news agency did not turn up a registration for ICG on government databases for NGOs or social enterprises.

Asked about Mr Kovrig, Canada said there was no "explicit indication" of any link to the Meng case. It said his case had been raised directly with Chinese officials.

The former Canadian resident was detained on 1 December in Vancouver, where she has family and property connections.

She was granted bail of C$10m (£6m; $7.4m) on Tuesday but could still be extradited to the US.

The US has been investigating Huawei, one of the world's largest smartphone makers, since 2016, believing that it used a subsidiary, Skycom, to bring US manufacturing equipment and millions of dollars in transactions to Iran in violation of sanctions.

The Supreme Court of British Columbia was told that Ms Meng, Huawei's chief financial officer, carried this out between 2009 and 2014.

She allegedly tricked US banks into clearing transactions for Huawei which were in reality for Skycom. The banks thus became "victim institutions" of fraud.

Lawyers for Ms Meng told the court that Huawei had already separated itself from Skycom.

Ms Meng faces up to 30 years in prison in the US if found guilty of the charges, the Canadian court heard.